In modern mass spectrometers, it is becoming more and more common to use ion sources which generate the ions in pure gases at atmospheric pressure. Electrospray ion sources are one example, but other types, such as atmospheric pressure MALDI (ionization by matrix-assisted laser desorption) have also become commercially available in the meantime. In these types of mass spectrometer with out-of-vacuum ion generation, the ions must initially be introduced into the vacuum system through apertures or capillaries together with a lot of gas; they must then be separated as far as possible from the gas and transported through various differential pump stages to the actual mass separating system, the mass spectrometric ion analyzer.
A combination of inlet capillary, first differential pump stage, skimmer, second differential pump stage and a multipole system for capturing the divergent ions behind the skimmer in the second differential pump stage has been adopted for this purpose, even though this system cannot capture anywhere near all the ions fed into the vacuum. Many ions are already lost in front of the skimmer.
In the first pump stage of the differential evacuation system of commercial mass spectrometers, the task of transferring the ions is undertaken almost exclusively by the stated combination of inflow capillary or inflow aperture with opposing skimmer. The skimmer is conical in shape, in order to deflect the impinging gas outwards, and has a central aperture for the passage of the ions into the next differential pump stage. A suction potential on the skimmer is intended to guide the ions as far as is possible to the central aperture. Many ions are lost at this stage, however, because they are entrained outwards in the outflow lobe of the gas and have no chance of reaching the central aperture in the skimmer to the next chamber.
An ion funnel arrangement has now been elucidated in U.S. Pat. No. 6,107,628 (R. D. Smith and S. A. Shaffer) which screens ions from a gas stream and accurately guides them to the aperture which leads to the next pressure stage of the differential pumping system. The ion yield is considerably higher than when skimmers are used. This ion funnel constitutes a special case of the more general embodiments of ion guide systems in U.S. Pat. No. 5,572,035 (J. Franzen).
The ion funnel consists of a packet of coaxially arranged apertured diaphragms separated by relatively narrow intermediate spaces and arranged with their surfaces parallel, the diameter of the apertures of the apertured diaphragms tapering more and more toward the central outlet hole into the next chamber. A funnel shape is thus formed in the interior of the ion funnel arrangement. The outer shape of the diaphragms usually is a square, with ceramic holding posts and ceramic spacers in the corners of the squares.
The gas is blown into the open funnel by the entrance aperture or by the gas capillary. The wall of the ion funnel is gas permeable because it is formed from the faces of the apertured diaphragms together with the intervening intermediate spaces. The gas escapes through the intermediate spaces between the apertured diaphragms and is pumped away by a vacuum pump. Only a very small amount of gas enters the next chamber of the differential pump arrangement through the very small outlet aperture. The apertured diaphragms are alternately subjected to both phases of an RF voltage (several hundred kilohertz to several megahertz, several hundred volts). This causes the internal wall of the funnel to repel the ions. The method of operation and effect of this repellent “pseudopotential” are described in detail in the cited patent specification U.S. Pat. No. 5,572,035. The pseudopotential prevents the ions from being entrained by the escaping gas stream through the intermediate spaces between the apertured diaphragms. The ions are screened. In addition, the apertured diaphragms are equipped with a stepped DC voltage (a few tens of volts) which utilizes the mobility of the ions to forcibly guide them through the strongly diluted gas in the ion funnel to the outlet hole.
The embodiment of the ion funnel, so far known by publications, is disadvantageous in a number of respects, however. On the one hand, the diaphragms are held by ceramic posts with spacer rings, and the spacer rings and the necessarily large diaphragm area obstruct the stream of escaping gas; the resistors and capacitors soldered onto the outside edge of the diaphragms represent a further obstruction. On the other hand, the ion funnel has a relatively large capacitance with relatively large dielectric losses, making it necessary to have a relatively powerful and hence expensive high frequency generator. Furthermore, the published embodiment has the disadvantage that it only admits a relatively narrow range of the mass-to-charge ratio m/z. The ratios of mass to charge m/z, which are the measured feature in mass spectrometry, are subsequently referred to as “specific masses” for the sake of simplicity.
The transfer of the ions into the next differential pump stages has long been undertaken by so-called ion guides, which normally have the form of radio-frequency carrying multipole systems, i.e. quadrupole, hexapole or octopole systems made of long, thin parallel pole rods. Other types of system have also been elucidated, for example a radio-frequency carrying double helix as described in the previously cited patent specification U.S. Pat. No. 5,572,035.